1. Field of the Invention
The present invention relates to a battery pack, to a method of determining malfunction that judges whether or not a current detection section, which detects rechargeable battery charging and discharging current, is operating abnormally, and to a malfunction decision circuit.
2. Description of the Related Art
State-of-the-art lithium ion battery charging (representative of rechargeable battery charging) charges batteries with a given current by constant current charging until the terminal voltage (referred to as battery voltage below) reaches a specified voltage set lower than the maximum allowable rechargeable battery voltage. Subsequently, charging switches to constant voltage charging. This is known as constant current-constant voltage charging and is the primary method in practice. If battery voltage exceeds the maximum battery voltage, battery lifetime and charging and discharging capacity degrade, and there is also risk of damage that even extends to battery combustion. Consequently, battery voltage is controlled during charging to avoid exceeding the maximum voltage.
Full-charge capacity is updated after each charge-discharge cycle based on the integral of the discharge current during discharge from full-charge to a voltage that halts discharging (revision-point voltage). The remaining capacity of a charged rechargeable battery is computed by subtracting the integral of the charging and discharging current (with positive current in the discharging direction) from the immediately prior revision of the full-charge capacity. Charging and discharging current is determined from the voltage drop across a current detection resistor connected in the charging and discharging path of the rechargeable battery. The computed remaining capacity is converted, for example, to a percentage of the full-charge capacity, and displayed to the operator.
When a high current flows that exceeds the rechargeable battery maximum allowable current, the result can extend to rechargeable battery failure such as over-heating, combustion, and explosion. For this reason, for example, the battery pack is provided with switching devices in the charging and discharging circuit path (refer to Japanese Laid-Open Patent Publication 2007-124,768). Here, when the voltage drop across the current detection resistor exceeds a specified voltage, the switching devices are switched OFF.
Accordingly, if for some reason the current detection resistor, which is indispensible for remaining capacity computation and over-current detection, fails due to a condition such as short circuit, not only is the ability to accurately determine remaining battery capacity lost, but also rechargeable battery safety is compromised.
In contrast, Japanese Laid-Open Patent Publication 2009-254,165 discloses a battery state detection circuit and battery pack that determines current detection circuit and/or voltage detection circuit failure. When rechargeable battery charging and discharging current detected by the current detection circuit, which has a current detection resistor, is effectively zero and the rechargeable battery terminal voltage detected by the voltage detection circuit is increasing, failure of either or both the current detection circuit and voltage detection is judged.
However, in the battery state detection circuit and battery pack cited in JP 2009-254,165, it cannot be determined if failure occurred in the voltage detection section or in the current detection section. In addition, the threshold for judging failure is not disclosed, and practical implementation is problematic.
The present invention was developed considering the situation described above. Thus, it is an object of the present invention to provide a method of determining malfunction that can reliably judge whether or not the current detection section has failed, and to provide a malfunction decision circuit and a battery pack.